python_requirements_info: don't overwrite results in 'mismatched' dict key (#4078) (#4111)

* bugfix: don't overwrite results in 'mismatched'

Whichever mismatched package is evaluated last is the value stored in the
'mismatched' key. Instead, it should have a subdict for each pkg that is mismatched
to keep in line with its documented usage.

* Update changelogs/fragments/4078-python_requirements_info.yaml

Co-authored-by: Felix Fontein <felix@fontein.de>

Co-authored-by: Felix Fontein <felix@fontein.de>
(cherry picked from commit 526369a243)

Co-authored-by: Will Hegedus <will@wbhegedus.me>

plugins/modules/system/python_requirements_info.py

@@ -193,7 +193,7 @@ def main():
                 'desired': dep,
             }
         else:
-            results['mismatched'] = {
+            results['mismatched'][pkg] = {
                 'installed': existing,
                 'desired': dep,
             }